The relevant issue here is the length of time this has been happening. This is common law private nuisance. It is literally what it says, a nuisance.
However, if you had just moved into the property you would have no cause of action because although it’s nuisance, you would be deemed to have moved to the nuisance.
As it is, you have been the property for 36 years and the nuisance has now come to you and hence, it is actionable and you can abate the nuisance.
I don’t accept what the owner says about fish swans and ducks as indeed you don’t.
If there were animals living along the bank and they have moved, that is further proof. Similarly, with the trees which would be a matter for the local authority particularly if they have tree preservation orders.
The environmental agency, and the rivers authority may be interested in the eroding bank.
He may need the water level high to work his mill, but he would only need it high occasionally and even then, if it causes nuisance, it’s a problem for him.
The way of dealing with this, apart from contacting the agencies I mentioned earlier with the evidence that you have is to get your solicitor to write to him threatening an application to court for an injunction to compel him to lower the level of water to that which it was at 2 years ago.
Hopefully, the threat coming from the solicitor and the threat of substantial legal costs will spur the owner into action but if it doesn’t, you are faced with applying to court for an injunction and asking the court to award costs against the mill owner.
Meanwhile, you need to gather as much evidence if you can to support your contention that the water is now firstly higher than it needs to be and secondly higher than it has ever been and thirdly, the fact that it is now damaging the environment and finally, meaning that you cannot enjoy your property as you could before.
Check your property insurance and contents insurance to see if you have legal expenses cover that would pay for the legal costs of taking the issue to court. If you have, pass this on to your insurer
Can I clarify anything else for you?
Please rate the service positive. It is an important part of the process by which experts get paid. It does not cost you anything but helps us greatly.